The complex bacterial masses which develop on and adhere firmly to teeth, collectively known as dental plaque, provide most of the causes of dental caries and periodontal diseases. In the adherence of plaque, bacterial polysaccharides are important but we are only beginning to understand the variety of molecules, their nature and their interactions which are responsible for this phenomenon. A thorough understanding of these molecular interactions should lead to innovations for the removal of plaque or the suppression of its development, and thereby should contribute importantly to the control of dental caries and periodontal diseases. Among the important interactions are numerous examples of specific coaggregation between bacteria of different species, e.g., Streptococcus sanguis and Actinomyces viscosus. S. sanguis is a very common, and often the most abundant bacterium identified in the early colonization of freshly cleaned teeth in man. Actinomyces, which are implicated in root caries and periodontal diseases, increase in number rapidly during the first several days of plaque formation. The coaggregation between these two may be important in the build-up of the Actinomyces population. The objective of this proposal is to provide an understanding of the molecular basis of coaggregation between S. sanguis 34 and A. viscosus T-14. Toward this goal we have: (1) Established conditions under which the coaggregation is reproducible and can be studied quantitatively, e.g., pH, specific ionic requirements, bacterial culture conditions, etc.; (2) Determined that the coaggregation requires the interaction between a protein on A. viscosus T14V with probably a carbohydrate on S. sanguis 34 and that this interaction is inhibited by lactose and certain other beta-galactosides specifically; (3) Obtained a crude, water-soluble, preparation from S. sanguis 34 which inhibits the coaggregation and, in this regard, is more potent than lactose. Specific aims of this proposal now are to purify and characterize the molecules from S. sanguis 34 which inhibit the coaggregation and to determine specific structural features which are essential for the inhibition.